validator with custom input field

textfield_validator.dart

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({
    super.key,
  });

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;

  const MyHomePage({
    Key? key,
    required this.title,
  }) : super(key: key);

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final GlobalKey<FormState> _formKey = GlobalKey<FormState>();
  final List<GlobalKey<FormFieldState>> _formFieldKeys =
      <GlobalKey<FormFieldState>>[
    GlobalKey<FormFieldState>(),
    GlobalKey<FormFieldState>(),
    GlobalKey<FormFieldState>(),
    GlobalKey<FormFieldState>(),
  ];

  static FormFieldValidator<String> _minLengthValidator(int length) {
    return (String? value) {
      if (value == null || value.length < length) {
        return 'fail';
      }
      return null;
    };
  }

  void _onSubmit() {
    if (!_formKey.currentState!.validate()) {
      return;
    }
  }

  bool _isValid() {
    for (GlobalKey<FormFieldState> key in _formFieldKeys) {
      final FormFieldState? state = key.currentState;
      if (state == null || !state.isValid) {
        return false;
      }
    }
    return true;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Form(
              key: _formKey,
              onChanged: () {
                setState(() {});
              },
              child: Column(
                children: <Widget>[
                  CSInputField(
                    key: _formFieldKeys[0],
                    autovalidateMode: AutovalidateMode.onUserInteraction,
                    validator: _minLengthValidator(5),
                  ),
                  Column(
                    children: <Widget>[
                      CSInputField(
                        key: _formFieldKeys[1],
                        autovalidateMode: AutovalidateMode.onUserInteraction,
                        validator: _minLengthValidator(5),
                      ),
                      CSInputField(
                        key: _formFieldKeys[2],
                        autovalidateMode: AutovalidateMode.onUserInteraction,
                        validator: _minLengthValidator(5),
                      ),
                    ],
                  ),
                  CSInputField(
                    key: _formFieldKeys[3],
                    autovalidateMode: AutovalidateMode.onUserInteraction,
                    validator: _minLengthValidator(5),
                  ),
                  ElevatedButton(
                    onPressed: _isValid() ? _onSubmit : null,
                    child: const Text('Submit'),
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}
class CSInputField extends StatefulWidget {
  CSInputField(
      {Key? key,
      this.hasLabel = false,
      this.hint,
      this.autovalidateMode,
      this.validator}):super(key:key);

  final bool hasLabel;
  final String? hint;
  final AutovalidateMode? autovalidateMode;
  String? Function(String?)? validator;
  @override
  State<CSInputField> createState() => _CSInputFieldState();
}

class _CSInputFieldState extends State<CSInputField> {
  @override
  Widget build(BuildContext context) {
    return TextFormField(
      keyboardType: TextInputType.visiblePassword,
      autovalidateMode: widget.autovalidateMode,
      validator: widget.validator,
    );
  }
}

created from https://dartpad.dartlang.org/?id=f626ef5b3b000c019f3228360d0c878a

_isValid() method returns false because state is null.